Generally, photoflash lamps responsive to high voltage pulse potentials, such as several hundred to several thousand volts available from a piezoelectric crystal for example, employ a bridge of primer material. Such flashlamps usually have a tubular glass envelope which is tipped off at one end and formed by a press seal at the opposite end. A pair of spaced electrical conductors are embedded into and pass through the press seal of the envelope and terminate therein in an ignition structure which usually includes a glass bead or reservoir of some type bridging the ends of the spaced electrical conductors. A primer material essentially covers, or at least extends between, the ends of the electrical conductors. Also, a filamentary combustible material is located adjacent the primer material within the envelope and a combustion-supporting gas under several atmospheres of pressure is contained within the envelope.
In operation, a high voltage pulse potential applied to the electrical conductors causes an electrical breakdown of the primer material. This breakdown or deflagration ignites the shredded filamentary combustible material to provide the desired light source.
Although the above-described photoflash lamp structures have been and still are utilized with varying degrees of success, there are a number of problem areas which are familiar to those skilled in the art. Perhaps the most prevalent of all problems with such structures is the tendency for the shredded combustible material to short-circuit the electrical conductors and deleteriously effect the desired electrical breakdown of the primer material.
An example of a known prior art lamp structure which attempts to overcome the above-described short circuiting conditions is set forth in U.S. Pat. No. 4,059,389, assigned to the Assignee of the present application. Therein, a pair of electrical conductors are sealed into one end of an envelope and the ends of the conductors within the envelope are melted and rounded to provide relatively smooth terminations. Thereafter, a layer of electrical insulating material, such as glass frit, is deposited onto the rounded ends of the electrical conductors and a portion of the conductors adjacent thereto. Then, a portion of the insulating material is removed from the rounded portions of the electrical conductors, and a coating of primer material is affixed to the rounded ends of the conductors covering both the insulated and the non-insulated portions.
Another known lamp structure utilizing a primer material within an envelope which also contains shredded combustible material and a combustion-supporting atmosphere is set forth in U.S. Pat. No. 4,097,220 issued to Cote et al. Therein, a pair of electrically conductive leads are positioned within an envelope and bent in a manner such that the ends thereof are in a single plane opposing one another. Thereafter, the ends and an adjacent portion of each of the electrically conductive leads are surrounded with a primer material.
One of the numerous problems associated with prior known high voltage flashlamp construction is that such structures are not particularly suitable to the newer subminiature photoflash arrays. Therein, the leads within the envelope must be relatively short because of the limited length of the envelope or the limited total lamp length. However, the shorter leads required by the shorter lamp present a whole list of additional problems. For example, the shortened leads within the envelope exhibit a tendency to seal over or become covered with glass during the envelope sealing process. Moreover, the shortened leads exhibit little, if any, resistance to glass flow which occurs during the above-mentioned envelope sealing. Further, the sharp and irregular cut of the shortened and trimmed wires easily penetrates the normal primer coating causing the shredded combustible material to short-circuit the primer material resulting in an inoperative lamp.